Most pulp and paper mills have chemical recovery boilers. The chemical recovery boiler is basically a steam generating power boiler that uses black liquor as its primary fuel. The two main objectives of the chemical recovery boiler are to recover the chemicals used in the cooking of wood and to use the thermal energy from combustion of the organics. The black liquor contains materials dissolved from the wood and spent chemicals from the digesting process. This liquor is washed from the pulp and concentrated in the evaporating plant before being sent to the recovery boiler. The recovery boiler converts the combustible materials from the wood into steam energy, converts sulfur compounds to sodium sulfide and converts sodium organic compounds to sodium carbonate. The chemicals are then discharged from the furnace bottom as molten smelt through smelt spouts for further processing.
Inside the recovery boiler, a molten bed of solids is present at the bottom of the boiler. The liquor is sprayed into the boiler approximately 20 to 35 feet above the bed level, depending upon the size and type of boiler at a height of. When the liquor is fired into the boiler, part of the liquor sticks to the walls and hardens while the rest either burns in suspension or falls to the bottom. The need arises for the boiler operator to be aware of the configuration and the height of the smelt bed for both economic and safety reasons.
It is also important for an operator to know what is happening in lime kilns, power boilers and cement kilns. Much of the discussion applicable to chemical recovery boilers is also applicable to kilns and power boilers.
The thermal system in a Kraft chemical recovery boiler is far more complicated than that found in other combustion systems, including kilns and power boilers. A Kraft recovery boiler acts simultaneously as a fuel dryer, a furnace and a chemical reactor. In Kraft recovery boilers, in addition to normal fossil fuel combustion products, there are also substantial amounts of hydrogen, hydrogen sulfide, sulfur dioxide, atomic and molecular sodium vapor, sodium hydroxide, methyl mercaptan, dimethyl sulfide and many other species. Reducing conditions prevail adjacent the smelt bed and these give way to oxidizing conditions higher in the boiler. In addition, there are substantial quantities of particulates suspended in the gases. These particles are predominantly molten droplets of sodium carbonate and sodium sulfate as well as unburned liquor droplets and burning char particles. It is important that an equilibrium, both physical and chemical, be maintained in the boiler to maximize chemical recovery.
Various techniques have been used to monitor boilers. U.S. Pat. No. 3,021,386 discloses a boiler port viewing system employing a TV camera having a vidicon pick up tube. Hamamatsu advertising literature dated Jan. 10, 1981, teaches using a TV camera equipped with an infrared vidicon tube in combination with a bandpass filter, and also discloses that an infrared TV camera can be used for temperature monitoring of smelting furnaces. U.S. Pat. No. 4,539,588 teaches using an infrared TV camera in combination with a bandpass filter for viewing a smelt bed in a Kraft recovery boiler.